Introduce ui.dll / libui.so for the component build.

ui/gfx/platform_font.h

Index: ui/gfx/platform_font.h
===================================================================
--- ui/gfx/platform_font.h	(revision 92815)
+++ ui/gfx/platform_font.h	(working copy)
@@ -11,12 +11,13 @@
 #include "base/memory/ref_counted.h"
 #include "base/string16.h"
 #include "ui/gfx/native_widget_types.h"
+#include "ui/ui_api.h"
 
 namespace gfx {
 
 class Font;
 
-class PlatformFont : public base::RefCounted<PlatformFont> {
+class UI_API PlatformFont : public base::RefCounted<PlatformFont> {
  public:
   // Create an appropriate PlatformFont implementation.
   static PlatformFont* CreateDefault();